Gas turbine engines (e.g. jet engines) include a rotating shaft having compressor and/or turbine blades mounted thereon and rotating therewith. Axial movement of the shaft relative to the remainder of the engine is considered to be an abnormal movement and indicative of engine failure (e.g. shaft breakage). Detection of axial movement of the shaft relative to the remainder of the engine can therefore be used to detect engine failure and used to prevent further damage to the engine by activating a shut off of the engine. A shaft links the turbine and compressor. If the shaft is broken, the turbine portion moves backwards because of the effect of combustion gases. The compressor elements would lose power and stop rotating.
It is known to detect abnormal movement of a gas turbine shaft relative to the engine casing by providing a circuit breaking element which is fixed to the shaft and moves therewith if and when the shaft moves in an axial direction. Movement of the circuit breaking element relative to the shaft breaks a circuit and thereby produces a signal.
U.S. Pat. No. 5,411,364 discloses an electro optic sensor for sensing unwanted or abnormal axial movement of turbine blades or rotors of a gas turbine. The sensing arrangement includes a pair of fibre optic wave guides interconnected through a frangible member disposed axially adjacent the turbine blades. Upon axial movement of the blades or rotors away from their normal position, the frangible element is broken to open the optical circuit associated with the wave guides. Associated electronic circuitry generates an output signal indicative of failure of the gas turbine rotor.
U.S. Pat. No. 6,607,349 discloses a broken shaft detection system and a method which uses a detector assembly mounted downstream of a power turbine wheel of a gas turbine engine to detect rearward axial motion of the wheel and thereby a broken shaft event. The detector assembly has a plunger positioned to be axially displaced against a link connected in an electrical circuit. The link may be broken when the plunger is displaced thereby creating an open circuit that may be detected by a detection and test element. The breaking may be communicated to an over-speed circuit that controls a shut off switch that interrupts fuel flow to the engine. The link may be connected to the detection and test element by two pairs of parallel wires to facilitate monitoring of circuit function and to detect failures that are not broken shaft event failures.
US 2007/0241921 discloses a frangible sensor element which is cut by a separating tang mounted on and moving axially with a gas turbine shaft when the shaft fails. The frangible sensor element includes a longish, mechanically severable sensor element, which is severed by the separating tang when this moves as a result of shaft failure. One embodiment of US 2007/0241921 has a circuit formed by two wires connected at the distal or free end of the sensor element by a resistor of a defined value, and another embodiment has a circuit in which two pairs of wires are looped or bent at the free or distal end of the sensor element to define a single continuous conductive path running from the proximal end of the frangible sensing element, to its distal end, then back to its proximal end before returning to its distal end and then returning to its proximal end.
A problem with the arrangement of US 2007/0241921 which relies on monitoring changes in current (and hence resistance) caused by the switch from a first resistive circuit to a second different resistive circuit is that the values of resistance vary with temperature. The resistance of the resistor elements themselves can vary significantly with temperature. Furthermore in sensors including circuits such as that described in US 2007/0241921, the wires are located within an insulating sheath (the conductive wires and insulating sheath being elements of a MI or mineral insulated cable as disclosed in WO 2007/028354) and the high operating temperatures (can be of the order of or exceed 800° C.) also significantly affect the insulation resistance of the cable. As jet engines and other gas turbines are high temperature environments, there is therefore a risk of a false alarm. This means that such sensors have small operating margins.
It is very important to avoid false alarms. Typically the sensor monitoring for shaft breakage is directly coupled to an engine cut-off to automatically and very quickly shut off the engine when the shaft breaks. False alarms therefore lead to the engine stopping when there is no need to do so. For a jet having two engines, turning one off unnecessarily has a clear risk.
In arrangements such as that disclosed in US 2007/0241921 and WO 2007/028354 which monitor signal amplitudes, the input signal amplitude is affected by many parameters (including supply voltage, amplifier gain, and resistance values). These parameters depend on, or are affected by, other parameters such temperature. It is difficult to keep a signal amplitude consistent over long operating periods as is required for effective and accurate operation. Keeping the input signal amplitude consistent over long operating periods requires special stabilization of power supply, special compensating resistors and a special design of amplifiers. This makes the measuring system expensive and complex.
As mentioned above arrangements such as those of US 2007/0241921 and WO 2007/028354 are also sensitive to the insulation resistance of the connected cable. It is well known that insulation resistance of mineral insulated cables varies with temperature and at high temperatures is very low affecting values of the measured resistance and this way potentially causing a false alarm.